After
a recent period of major rainfall, I visited a wooded area to see what
animals I could find, wondering if any had left for Noah's Ark. I turned
over an old piece of roofing tin and noticed that the ant colony beneath
had their eggs stacked on the surface, presumably to protect them from
rising waters.

Ants are
incredible creatures. Of the more than 15,000 species of these highly
social insects (that includes some yet to be described by scientists),
those that have been studied have displayed extraordinary behaviors.
Watching how ants had responded to the dramatic change in environmental
conditions to protect their reproductive effort reminded me of another
behavioral response of ants related to reproduction of a tree.

When insects
benefit from getting nectar from a flower, they inadvertently perform
an essential function for the plant: pollination. Such mutually beneficial
relationships between plants and animals are fascinating and ecologically
complex, and understanding how they work can be a scientific challenge.
Research on ants and acacia trees (members of the mimosa family) revealed
a special alliance in which each helps the other survive.

Many fuzzy-looking
acacia flowers are fragrant, a lure to pollinating insects, and also
have ants that live on them and protect the trees from plant-eating
insects. The ants attack an invading beetle or aphid that plans to make
a meal of acacia leaves. In return, the tree provides shelter for the
ants, which also thrive on nutrients produced by the tree. A mystery
surrounding the observed bond between acacias and ants is how other
insects manage to pollinate the flowers when the tree is guarded by
ants. How does the ant distinguish between a food-searching insect that
would harm the tree from one seeking nectar that will benefit the tree?

One study
focused on ants protecting African acacia trees whose flowers are pollinated
mainly by bees during the midday period. Like any other insect, a bee
is not welcomed by ants guarding a tree. Ants were observed to protect
the buds and older flowers from insects in the same manner that leaves
were protected. The guarding ants did not permit any insect to stay
in the vicinity of the plant before flower development and during seed
production. However, once the young flowers were ready to be pollinated,
the ants began to avoid the area around the flower, allowing bees and
other insects to gather nectar and serve as pollinators.

As each
flower aged, however, the guarding ants began to march back onto the
scene. The scientists concluded that when the flowers are ready to be
pollinated they produce a chemical that acts as an ant repellent. To
test their hypothesis, they picked newly developed flowers at a stage
ripe for pollination and wiped some of the old flowers with them. The
ants present around old flowers, which would normally be protected,
retreated from those wiped with new flowers. The behavior of the ants
supported the hypothesis that a chemical produced by the flower temporarily
deterred ants and allowed bees to pollinate the flowers without being
attacked.

One interpretation
of how the acacia-ant system developed is that a genetic change occurred
in the ancient past so that some acacia trees began to produce a chemical
that kept ants away from the flowers during pollination. Acacias without
the genetic change of an ant-deterring chemical would still be pollinated
but at a lesser rate than trees with the genetic change that would result
in their becoming much more prolific. Nearly every flower would get
pollinated and thus more seeds and, ultimately, more trees would be
produced. Eventually, the trees most likely to have offspring would
possess the genetic makeup to produce the chemical.

So how
does an ant know not to attack during pollination season? In one sense,
the ant doesn't know, but the tree does. Such intricate ecological systems
working in favor of two species are intriguing, and ants continue to
be remarkable creatures.